In the petroleum refinery processes for producing various fuel oils such as gasoline, diesel and kerosene, etc., the sulfur contained in the crude oils is removed as hydrogen sulfide gas by hydrodesulfurization process. The highly toxic hydrogen sulfide gas is then converted to elemental sulfur in sulfur-recovery plants or so-called Claus plants. The Claus plants can be blamed for part of the hydrogen sulfide emissions, as the sulfur-recovery rate is 90-98% depending on the number of reactors used therein. During the last two decades a great number of Claus tail-gas treating (TGT) processes have been developed to increase the total sulfur-recovery efficiency. Conventional Claus TGT processes involve a hydrogen sulfide absorption step, in which a tail gas containing unreacted hydrogen sulfide is introduced into an alkaline bath. Removing the last percentages of sulfur by means of these conventional Claus TGT processes is expensive, both in terms of capital investment cost and energy consumption. In order to avoid the shortcoming of these solution-absorption type Claus TGT processes, a dry type Claus TGT process has been developed in Netherlands which comprises recovering elemental sulfur from the Claus tail gas by selective oxidation of hydrogen sulfide in the presence of a catalytically active mixture of iron and chromium oxides deposited on an alpha-alumina support Oil and Gas Journal, 86(41), p. 68-71, (Oct. 10, 1988); Catalysis Today, 16, p. 263-271, 1993!. The dry type Claus TGT process is simple and economical; however, the chromium atom contained in the catalyst is a toxic substance.
U.S. patent application serial No. 08/523,362, filed Sep. 5, 1995, now allowed to be patented, discloses an iron-based catalyst for recovering elemental sulfur by selectively oxidizing hydrogen sulfide to elemental sulfur. The iron-based catalyst comprises a component selected from cerium, tin, antimony and molybdenum as a promoter. The selectivity of the iron and molybdenum mixed-oxide catalyst is sacrificed, although the conversion thereof is enhanced in comparison with the iron oxide catalyst.
The primary objective of the present process is to provide a process for recovering elemental sulfur from a gas mixture containing hydrogen sulfide by selective oxidation of hydrogen sulfide in the presence of a mixed-oxide catalyst, which has an improved elemental sulfur yield.